The present invention relates to a multi-magnification copier and more particularly to a copier which includes an imaging system which anamorphically produces reduced or enlarged copies of an original document
It is well known in the prior art to produce reduced or enlarged copies of original documents by an anamorphic process, e.g. by introducing a velocity mismatch between a document scanning optics in a photoreceptor moving in a process direction upon which the latent image is formed. For anamorphic reduction, the scanning is accomplished at a faster rate than a photoreceptor motion resulting in the image being compressed in the process direction. For anamorphic enlargement, the scanning is accomplished at a slower rate than the photoreceptor motion, thereby elongating the image in the process direction. U.S. Pat. No. 4,897,688 (Smith et. al.) discloses an extensive review of prior art anamorphic techniques and the contents of this patent is hereby incorporated by reference. The Smith et. al. patent usefully summarizes the critical problems which must resolved in a practical anamorphic magnification copying system. The anamorphic magnification creates a distortion of the image which necessitates a very narrow imaging slit at the photoreceptor surface. A narrow slit width in turn requires increased exposure of the document being scanned. The Smith et. al. patent provided an enablement of an anamorphic machine by forming a narrow effective slit width by using a gradient index lens in combination with a field stop attached to the side of the lens, the field stop defining a narrow slit. Document illumination was optimized by introducing a cylindrical reflector on the opposite side of the illuminated scan strip area to concentrate light from the scan illumination source. While this design results in an effective slit width of about one millimeter, resolution goals have further increased, for example, at least 2.5 lines per millimeter are current requirements for high quality output copies. To reach this higher resolution, it has been found that an even narrower imaging slit than that possible with the prior art is required. Another problem with the Smith et. al. design is that slit aperture near the photoreceptor attracts dirt and toner builds which bridge the aperture and tend to obscure part of the final image.
The present invention is, therefore, directed to an anamorphic imaging system which forms images on a photoreceptor surface through an imaging slit having a less than one millimeter width while maintaining illumination exposure levels. In one embodiment, the narrow effective slit width is enabled by attaching blockers to the sides of a gradient index lens array which transmits the image from the object to the image plane. The blockers cooperate to provide an effective narrow slit of &lt;1 mm between them. This embodiment also substantially reduces the direct and toner contamination by removing the mechanical aperture from the vicinity of the photoreceptor. In another embodiment, a narrower slit width is accomplished by preparing a photographic image of a narrow (&lt;1 mm) slit and placing the film on or beneath the object plane. Illumination exposure is maintained in a still further embodiment by using a faster lens array and compensating for reduction in total conjugate by adding an extra pitch to the lens array. Relevant publications for this embodiment are an article in Applied Optics, Vol. 21, No. 15, Aug. 1, 1982 entitled "Optical properties of GRIN fiber lens arrays: dependence on fiber length" by William Lama, an article in SPIE Vol. 935 Gradient-Index Optics and Miniature Optics (1988) entitled "Office applications of gradient-index optics" by James D. Rees and a disclosure in the Xerox Disclosure Journal, Vol. 12, No. 3, May/June 1987, entitled "Method to Lengthen the Total Conjugate of Fast Gradient Index Arrays" by James D. Rees. All of these publications are hereby incorporated by reference.
More particularly, the present invention relates to an improved copier for producing anamorphically reduced or enlarged images of a document moved across a surface of a document platen at a scanning speed, the combination of: an illumination means for providing a narrow band of illumination along a scanning strip of the bottom surface of the platen, means for moving an original document at a scanning speed across the top surface of the platen so that the document is scanned by incremental illumination along said scan strip, linear lens means positioned along an optical path extending between said platen and a photoreceptor moving at said process speed, said linear lens means projecting light reflected from said document during said incremental scanning, and means for changing the scanning speed relative to the process speed, the improvement wherein the light transmitted through said lens means onto the photoreceptor surface passes through an effective slit width less than 1 mm.